Backlights are used to illuminate liquid crystal displays (LCDs). LCDs with backlights are used in small displays for cell phones and personal digital assistants (PDAs) as well as in large displays for computer monitors and televisions. Often, the light source for the backlight includes one or more cold cathode fluorescent lamps (CCFLs). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (ELP), or one or more hot cathode fluorescent lamps (HCFLs).
The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, they require adequate idle time to ignite, and they require delicate handling. Moreover, LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. Because of this, displays with LED backlights can consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings. FIG. 1 shows an exemplary flat panel display 10 with a backlighting system having three independent strings of LEDs 1, 2 and 3. The first string of LEDs 1 includes 7 LEDs 4, 5, 6, 7, 8, 9 and 11 discretely scattered across the display 10 and connected in series. The first string 1 is controlled by the drive circuit 12. The second string 2 is controlled by the drive circuit 13 and the third string 3 is controlled by the drive circuit 14. The LEDs of the LED strings 1, 2 and 3 can be connected in series by wires, traces or other connecting elements.
FIG. 2 shows another exemplary flat panel display 20 with a backlighting system having three independent strings of LEDs 21, 22 and 23. In this embodiment, the strings 21, 22 and 23 are arranged in a vertical fashion. The three strings 21, 22 and 23 are parallel to each other. The first string 21 includes 7 LEDs 24, 25, 26, 27, 28, 29 and 31 connected in series, and is controlled by the drive circuit, or driver, 32. The second string 22 is controlled by the drive circuit 33 and the third string 23 is controlled by the drive circuit 34. One of ordinary skill in the art will appreciate that the LED strings can also be arranged in a horizontal fashion or in another configuration.
An important feature for displays is the ability to control the brightness. In LCDs, the brightness is controlled by changing the intensity of the backlight. The intensity of an LED, or luminosity, is a function of the current flowing through the LED. FIG. 3 shows a representative plot of luminous intensity as a function of forward current for an LED. As the current in the LED increases, the intensity of the light produced by the LED increases. Therefore, the current in the backlight strings must be controlled and be stable in order to control and maintain the backlight intensity.
To generate a stable current, circuits for driving LEDs use constant current sources. A constant current source is a source that maintains current at a constant level irrespective of changes in the drive voltage. FIG. 4 is a representation of a circuit used to generate a constant current. The operational amplifier 40 of FIG. 4 has a non-inverting input 41, an inverting input 42, and an output 43. To create a constant current source, the output of the amplifier 40 may be connected to the gate of a transistor 44. The transistor 44 is shown in FIG. 4 as a field effect transistor (“FET”), but other types of transistors may be used as well. The drain of the transistor is connected to the load, which in FIG. 4 is an array of LEDs 45. The inverting input of the amplifier 40 is connected to the source of the transistor 44. The source of the transistor 44 is also connected to ground through a sensing resistor RS 46. When a reference voltage is applied to the non-inverting input of the amplifier 40, the amplifier increases the output voltage until the voltage at the inverting input matches the voltage at the non-inverting input. As the voltage at the output of the amplifier 40 increases, the voltage at the gate of the transistor 44 increases. As the voltage at the gate of the transistor 44 increases, the current from the drain to the source of the transistor 44 increases.
FIG. 5 illustrates a typical relationship between the source current and the gate voltage for an exemplary transistor. Since little to no current flows into the inverting input of the amplifier 40, the increased current passes through the sensing resistor RS 46. As the current across the sensing resistor RS 46 increases, the voltage drop across the sensing resistor Rs 46 increases according to Ohm's law: voltage drop (V)=current (i)*resistance (R). This process continues until the voltage at the inverting input of the amplifier 40 equals the voltage at the non-inverting input. If, however, the voltage at the inverting input is higher than that at the non-inverting input, the voltage at the output of the amplifier 40 decreases. That in turn decreases the source voltage of the transistor 44 and hence decreases the current that passes from the drain to the source of the transistor 44. Therefore, the circuit of FIG. 4 keeps the voltage at the inverting input and the source side of the transistor 44 equal to the voltage applied to the non-inverting input of the amplifier 40 irrespective of changes in the drive voltage VSET.
One of the limitations of the constant current source of FIG. 4 is that it is not readily scalable. For a given input voltage on the non-inverting input of the amplifier 40, the only way to adjust the source current and hence the current in the load is to change the resistance of the sensing resistor 46. Variable resistors or potentiometers are prohibitively expensive and large. Changing the sensing resistor 46 to scale the current is not practical for many applications.
Another limitation of the constant current source of FIG. 4 is that it is increasingly inefficient at higher currents. When current passes through the sensing resistor 46, power is dissipated according to the following relationship: power dissipated (P)=current2 (i2)*resistance (R). Therefore, at increased currents, a larger amount of power is dissipated in the sensing resistor RS 46.
In the prior art, if the sensing resistor is integrated inside the integrated circuit, then there are problems with current source accuracy due to temperature changes. As power is dissipated, the temperature of the sensing resistor increases. As the temperature of the resistor changes, the resistance of the resistor changes unless the resistor is a zero thermal coefficient resistor. As the resistance of the sensing resistor changes, the current in the load changes according to Ohm's Law. Most foundry processes do not use a process that can generate a resistor with zero thermal coefficient behavior. A few processes can fabricate thin film resistors with a temperature coefficient close to zero, however these processes add cost and complexity to the integrated circuit fabrication process.
For incorporation into integrated circuits, a further limitation of the constant current source of FIG. 4 is that the surface area of the sensing resistor RS 46 may be inconveniently large for many applications. For example, if the voltage at the non-inverting input of the amplifier 40 is 150 mV and the desired source current is 20 mA, the resistance of the sensing resistor RS 46 must be 150 mV/20 mA=7.5%. The length (L) of the resistor divided by the width (W) of the resistor equals the resistance of the resistor divided by the sheet resistance RSH. That is, L/W=7.5Ω/RSH. Assuming the contact resistance is negligible and the resistor is made of a metal with a sheet resistance RSH of 60 mΩ/□, then L/W=7.5Ω/60 mΩ/□=125. If the contact density of the chip used for the constant current source is 0.5 mA/contact, then the number of contacts will be 20 mA/0.5 mA, or 40. Assuming the contact width is 0.4 μm and the space between each contact is 0.7 μm, then the total width required for contacts is 44 μm. Since L/W equals 125 above, L equals 125*44 μm. So L equals 5,500 μm. This rough calculation indicates the sensing resistor 46 may be 242,000 μm2. This is a significant amount of the space on a typical semiconductor chip.
The resistor surface areas required by the previous designs are impractical for integrated circuits in high-current applications. The present invention overcomes many of the limitations of the prior art current sources through innovative systems and methods for providing a constant current source that is scalable and efficient.